Recent work on the development of conceptual knowledge and intuitive theories in normally developing children has identified certain points in the natural learning path which, although necessary for development to occur, nonetheless impose relative conceptual bottle necks on the learning process. These conceptual bottlenecks occur when a theory that the child is trying to acquire (e.g. the adult's biological theory) does not map directly into concepts in the child's existing theory. Learning that involves mismaps of this sort requires deep conceptual change on the part of the child; conceptual change that is low, slow, and mentally difficult to achieve compared to learning which does not involve these mismaps. Williams syndrome is a neurodevelopmental disorder of genetic origin that leads to mental retardation. One result of this retardation seem to include the inability to spontaneously negotiate these conceptual bottle- necks the way that normally developing children do, at least in the domain of intuitive biology. The aim of this project is to gather more information on the scope and impairment in WS. Adults and adolescents with WS will be tested for their conceptual achievements in the domain of number, specifically with regard to number of concepts implicated in conceptual change, i.e., cardinality, zero, infinity, and conservation. Their performance will then be compared with that of normally developing children matched for verbal mental age. A better characterization of the underlying impairment in WS will pave the way for a clearer understanding of the cognitive mechanisms involved in conceptual change and the possible avenues for intervention and remediation and impaired populations.